Measurement of the size of particles in a particulate mass is required in a great many industrial operations. Specifically, the minerals industry needs to know when an ore has been sufficiently crushed. To build highways and large buildings, the soil must be analyzed for stability, one factor of which is particle size. Many products, notably pigments, have to be ground to required particle sizes.
Much of this particle sizing is now accomplished manually by having a technician expose samples of the powder to a series of sieves -- pans with woven wire bottoms -- from which, by weighing the amounts passing and retained, he can calculate percentages of particles having certain sizes. The work is tedious and time consuming. Many industrial materials are processed wet. This means that a sample has either to be dried before sieving or sieved and then the fractions dried in order to weigh them accurately.
Basically, sieving a powder to obtain its particle size distribution is quite simple when performed manually. It is a tedious and time consuming procedure, nevertheless. A series of sieves -- usually pans with wire grid bottoms -- are arranged vertically one above the other with the coarsest grid on top and the finest on the bottom. The powder in question is poured on the top sieve, and the sieves are shaken or vibrated until the particles distribute themselves in discrete fractions on the sieves. The fractions and the powder passing the finest sieve are weighed individually and the mass percentage retained (or passing) is calculated in terms of the sieve size openings.
The test may be performed with either a wet or a dry powder. In dry testing, complete dryness is required to eliminate moisture-formed aggregates, but frictional effects often give rise to electrostatic charges which cause the particles to adhere to one another and to the sieves, both retaining frame and wire grid. Some particles are usually lost by dusting also. Wet sieving requires repeated flushing with fresh water which means that if fractional sizes are to be determined, the portion of the powder passing all sieves must be captured by filtering or that the initial powder mass must have been determined, which usually means that another complete sample has to be evaporated to dryness. The fractions collected on each sieve must also be dried and then weighed in order for mass percentages to be calculated. Handling the finer sieves in either wet or dry sieving poses a problem because they are quite easily damaged.
Sieving has great industrial appeal as a means for size analysis despite its disadvantages because it gives a direct measurement, the basis for which is quite evident. In present practice, results are subject to considerable error because of losses due to handling, uncertainties of drying and weighing, incomplete separation, electrostatic effects (when dry), damaged sieves, and the like. Often only one fractionation, i.e., one separation into those particles larger and those smaller than a certain sieve size, is employed in control procedures in the interest of saving time and reducing manual effort.
Various mechanisms have been provided in order to improve upon the above purely manual technique. For example, a set of sieves each of a predetermined standard weight may be employed to avoid the necessity for transferring each fraction from its sieve to a weighing pan. Each sieve with its fraction is weighed directly (after drying, in the case of wet sieving). Various mechanical shakers have been provided for the sieves, including a type which shakes the sieves one at a time. This relieves the technician of the task of manually shaking the sieves.
Sieve separation as described above is practiced for reasons other than size measurement. Sometimes the reason is to learn through subsequent chemical analysis how composition changes with particle size. Such information is necessary in mineral recovery operations where the need is to determine how fine to grind for optimum mineral release. At other times it is to establish the most desirable particle size for a material in subsequent use, for example as a filler in a floor tile or cover. The present invention permits ready fractionation of powders into discrete size ranges for these purposes also.